In a wireless local area network (WLAN), a single central base station, e.g., an access point (AP) may communicate with multiple mobile stations (STA) over a wireless communication link in what may be referred to as point to multi-point communication. For example, the AP may utilize a time domain duplexing (TDD) channel access scheme, in which transmissions to the multiple stations may be multiplexed in different time slots in the same frequency band, or a frequency domain duplexing (FDD) channel access scheme, in which transmissions to the multiple stations may occur simultaneously, but in different frequency bands. Thus, although an AP in a WLAN may potentially communicate with multiple users, in many cases, for example, in TDD and/or FDD systems, the communication is point to point at any single instance of time and frequency.
Spatial division multiple access (SDMA) is a method of multiplexing several signal streams, each one targeted to a different destination, simultaneously, by utilizing multiple antennas. An SDMA channel access method may enable the use of the same frequency at the same time to communicate with several stations located in different places. For example, an SDMA AP having multiple antennas may use a beamforming technique to transmit to several remote stations simultaneously. Each transmit antenna may transmit the intended signal multiplied by a certain weight, and by dynamically controlling the weights of each antenna the transmission may be directed to a desired location. Under certain assumptions, it can be shown that data transmissions to N users can be multiplexed together using N antennas, for a total capacity increase by a factor N compared with simple legacy networks that allow access to the wireless medium for only a single user at a time.
However, the integration of higher capacity transmission technology into existing wireless LANs may require operation in accordance with the existing systems' physical layer (PHY) and media access control layer (MAC) protocols, e.g., for backwards compatibility. For example, the MAC protocol may ensure that all users have an equal opportunity to contend for access to the medium, provide means for avoiding collisions, e.g., due to concurrent transmissions by two or more stations, and provide a method of recovery from collisions.
The Institute of Electrical and Electronics Engineers (IEEE) 802.11 family of standards (“IEEE-Std 802.11, 1999 Edition (ISO/IEC 8802-11: 1999)” and derivatives thereof) provides one current MAC protocol for WLAN systems. For example, the IEEE 802.11 MAC may regulate access to the wireless medium by equal priority for access contention, e.g., using a collision sense multiple access/collision avoidance (CSMA/CA) scheme, in which each station implements a carrier sense mechanism to detect the state of the wireless medium, and a positive acknowledgement scheme to ensure correct reception of data frames.
Backward compatibility of APs with user stations operating on earlier, slower versions of a transmission standard may reduce overall throughput. For example, in the IEEE 802.11g standard the throughput may reach 54 Mbps. However, in a deployment scenario having legacy stations designed to an earlier standard, e.g., IEEE 802.11b, that may communicate at less than 11 Mbps, the legacy stations may dominate the usage of the wireless medium to the detriment of user stations of more recent design. This problem may be further compounded as new standards such as, e.g., the IEEE 802.11n multiple-input-multiple-output (MIMO) standard which allows for data rates over 100 Mbps, are deployed.